288. Upon the submission of proper and reasonable estimates of costs and expenses to be incurred by Exolum, the undertaker must pay Exolum sufficiently in advance but to enable Exolum to undertake its obligations under this Schedule provided that in the event that the costs reasonably incurred by Exolum are less than the amount paid by the undertaker pursuant to this paragraph 288 then Exolum must promptly repay any overpayment to the undertaker within 30 days of the payment of those costs.